The increasing number of frequency bands approved for modern cellular phones and other wireless applications has the effect that the individual radio systems are moving closer and closer together in the frequency spectrum. Cleanly separating adjacent frequency bands requires steep-edge filters, i.e. filters whose passband in the direction of an adjacent passband of a different radio system rapidly undergoes transition to a sufficient damping.
Besides closely adjacent radio bands which have to be separated from one another in the telephone terminal, a further problem that arises in the filters is production-dictated frequency variations which make it even more difficult to cleanly separate adjacent frequency bands. In this regard, in modern production standards, in general, a manufacturing tolerance in the range of 800 to 1500 ppm should be expected, which can affect both sides of the passband. In the worst case, therefore, the gap between two adjacent frequency bands is narrowed from both sides by unfavorably positioned manufacturing tolerances.
A further frequency shift arises in the case of temperature changes since the resonant frequency of acoustic resonators such as are usually used in filters changes with temperature. This change is described by the so-called TCF (Temperature Coefficient of Frequency) in ppm/K. The TCF of a BAW resonator e.g. is dependent on the temperature coefficients of the acoustic parameters (speed of sound and impedance) of the respective materials from which the resonator is constructed. To a small extent it is also dependent on the coefficient of thermal expansion of the materials. The TCF of the overall resonator results as a type of weighted average value of the properties of the individual layers, wherein the weighting is proportional to the local stress in the component especially in the BAW resonator. Regions having high stress should be weighted more highly than regions having low stress. The effective TCF is thus determined by the detailed construction of the layer stack in the resonator.
Cellular phones are usually specified to a temperature range of −35 to +85° C. Within this temperature interval a frequency margin of approximately 3000 ppm results for the acoustic resonators used in filters having a typical TCF of −25 ppm/K. A filter which takes account of this frequency margin has to be implemented with an extremely steep-edged design in the case of some frequency band combinations since this frequency margin claims a considerable proportion of this frequency gap for itself. In this regard, for example, band 22 has a frequency gap of only 5700 ppm between the TX band and the RX band.
Considerable efforts are therefore being made to reduce or even completely compensate for the temperature coefficient. For this purpose it is known to use acoustic resonators composed of materials which have a positive temperature coefficient of their viscoelastic properties. Upon heating, these materials exhibit an increase in their modulus of elasticity, as a result of which the resonant frequency increases. Since the majority of all materials used in resonators have a negative temperature coefficient of their viscoelastic properties, the temperature coefficients of the different materials can mutually compensate for one another by means of a suitable combination of materials.
BAW resonators constructed directly as a layer stack on a substrate material, so-called SMR-type BAW resonators (SMR=solidly mounted resonator), are usually constructed on an acoustic mirror in which high and low impedance layers alternate and form a Bragg mirror. A Bragg mirror then has an optimum reflection for an acoustic wave having the wavelength λ if the individual low and high impedance layers have a thickness of in each case approximately λ/4.
One possibility for temperature compensation consists in increasing the thickness of the topmost mirror layer of a Bragg mirror, said layer being directly adjacent to the bottom electrode of the resonator and usually consisting of SiO2, beyond the abovementioned standard value of λ/4. This leads to higher stress densities in the SiO2 layer and thus also to a better temperature compensation, although also to a reduction of the resonant frequency. The latter in turn can be compensated for by a reduction of the layer thickness of the piezoelectric layer of the resonator. Although this achieves a compensation of the TCF, the reduction of the layer thickness of the piezoelectric layer as a consequence also greatly reduces the piezo-coupling, e.g. by approximately 50%.
Alternatively, according to the prior art it is possible to arrange a silicon dioxide layer between the electrodes and the piezoelectric layer and even between sublayers of the piezoelectric layer. In this case, although the degradation of the effective coupling is not as pronounced, it nevertheless reaches approximately 30%. This has the effect that such filters with compensation of the TCF can no longer fulfill the required specifications in many filter applications in the mobile radio sector.